﻿public class Solution {
    public int NumTrees(int n)
    {
        if (n <= 1) return 1;
        
        int[] dp = new int[n + 1];
        dp[0] = dp[1] = 1;

        for (int i = 2; i <= n; i++)
        {
            for (int j = 0; j < i; j++)
            {
                dp[i] += dp[j] * dp[i - j - 1];
            }
        }
        return dp[n];
    }

    public static void Main(string[] args)
    {
        int n = 3;
        Solution solution = new Solution();
        int res = solution.NumTrees(n);
        Console.WriteLine(res);
    }
}